1. Field of the Invention
This invention relates to the improvement of a fluorescent character-indicating tube wherein a plurality of co-planar anode segments are arranged in a predetermined form and more specifically to an improved tube of the character described having an improved structure of the anode segments.
2. Description of the Prior Art
A fluorescent character-indicating tube is generally used for the display of an electronic calculator, and presently, so called multi-digit type fluorescent character-indicating tubes are popularly used. In a multi-digit type tube, a plurality of co-planar digit elements are arranged on a base plate. One digit element includes a plurality of anode segments arranged in a predetermined form on a base plate.
There are two methods for making the anode segment structure of this type tube.
One is to use a base plate in which a plurality of grooves are arranged so as to indicate some indicia, and a phosphor material is disposed in it by a process of sedimentation. Of course, before this step, an anode electrode segment is arranged on a bottom surface of a groove. This grooved base plate type structure is advantageous to obtain a sharply outlined anode segment. However, it requires rather complex steps and many parts to form such a grooved base plate with anode electrodes in it.
For avoiding these complex structure and reducing manufacturing costs the other method, that is. The printing method is preferred. In this case, both anode electrode segments and phosphor segments are arranged by a screen printing process, respectively. According to this method, manufcaturing cost can be reduced. But another technical problem occurs in this printing method. That is printing or alignment differences between anode electrode segments and phosphor segments. This "shear" in printing is inevitable even if the same original screen pattern is used for printing both anode electrode segments and phosphor segments.
The amount of these differences in printing is generally about 130 to 300.mu. where the total printing length is about 60mm. Therefore, if the printing method is used, it is necessary to leave a space of at least about 400.mu. between the anode electrode segments. Consequently, this spoils the continuity of the digit pattern.
So in order to avoid this defect, in printing, it is presently proposed to make the phosphor segment smaller than the anode electrode segment for leaving a spacing tolerance. However, in doing so, the brightness of the tube is decreased because a fair number of electrons generated are attracted to the exposed surfaces of the anode electrode of which the electric potential is higher than the surface of the phosphor segment. Accordingly, decreased brightness appears particularly at the edge portions of the phosphor segment. Consequently it becomes impossible to obtain uniform brightness in indicated characters.